The present invention relates to stabilized aqueous dispersions of gypsum and with the use thereof as filling materials and coating pigments in the production of paper and cardboard.
Aqueous dispersions of finely-divided gypsum (CaSO.sub.4.2H.sub.2 O), chalk (CaCO.sub.3) or kaolin (Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O) are employed as filling materials and as coating masses in the production of paper and cardboard. However, aqueous dispersions of gypsum have found only limited use since it is difficult to produce slurries with a sufficiently high content of gypsum which simultaneously display a low viscosity and the requisite high stability necessary for further use. For further use in the case of such dispersions, there is normally required a dry material content of 60 to 80% by weight of solids, as well as viscosities of below 300 mPas, and preferably below 200 mPas. It is further required that the dispersions, even after standing for a comparatively long time, form only a small amount of sediment which can be completely redispersed upon stirring, or preferably form no sediment at all.
As gypsum pigment, either finely-ground natural gypsum or a gypsum obtained in the chemical industry as a waste product can be used. Suitable for use is gypsum obtained from the desulfurization of flue gases and from the production of phosphoric acid, lactic acid or citric acid. Because of its high degree of purity and homogeneity, chemical gypsum is of increasing importance.
For use as pigments, these products are dry or wet ground to particle sizes of less than 20 .mu.m., and preferably to sizes of less than 10 .mu.m. 50% of these particles must be below 2 .mu.m., and in many cases it is necessary that 70% be smaller than 2 .mu.m.
Since aqueous dispersions containing dry material contents of about 70% are solid cakes without further additives, it is necessary to plastify these mixtures by means of additives. For this purpose, it has long been conventional to add dispersion agents such as carboxymethylcellulose, polyacrylates or appropriate anionic, surface-active compounds. It is presumed that these anionic compounds deposit on the surface of the fine pigment particles and, due to their charge, bring about an electrostatic repulsion of the various particles from one another.
In addition, it is typical to add to the slurries low molecular weight organic or inorganic complex formers. Inorganic phosphates such as pyrophosphates, hexametaphosphates or phosphonates are especially preferred.
In addition to the above-mentioned surfaceactive anionic compounds and complexing agents, the slurries may also contain conventional additives such as bactericides, defoaming agents, pH regulators and stabilizers.
In some cases, the stabilization achieved with the dispersion agents alone can be improved with the addition of supplementary stabilizers. Stabilizers such as clays and viscosity-increasing agents such as cellulose ethers, for example carboxymethylcellulose, methylcellulose and hydroxyethylcellulose, have proved to be useful. The said stabilizers are added in an amount of from 0.02 to 0.5, and preferably in an amount of from 0.05 to 0.3% by weight of the slurry.
In many cases, it is advantageous to use impure gypsum in a mixture with chalk or kaolin. By doing so, improved pigment actions can be achieved. The mixture can be produced by adding a dry grinding of one component to an aqueous slurry of the other, or by mixing together two aqueous slurries of the two components.
With all of these slurries it has been a problem to attain a sufficiently high solid material content, and to keep the viscosity low enough so that the products can be worked up satisfactorily and are stable upon storage, i.e. do not form a sediment.
From the Federal Republic of Germany Patent Specification No. 36 12 087, it is known that a surface-active alkylbenzenesulphonic acid, in combination with an inorganic phosphate or an organic amine compound as a complexing agent in an amount of from 0.5 to 2% of the solid material content, can produce a dispersion of gypsum or chalk with a dry material content of 65 to 75%. Viscosities of 140 to about 1000 mPas, measured in a Brookfield viscosimeter, are thereby attained. For many purposes, however, this viscosity is too high. Furthermore, after 24 hours a sediment separates out from these dispersions which cannot be fully redispersed again.
From British Patent Specification No. 2,179,956, it is known to suspend kaolin or chalk in admixture with gypsum and to add starch and polyacrylates as dispersion agents. Besides carboxymethylcellulose (CMC), other dispersion agents which can be added include acidic phosphoric acid esters which are esterified with alkyl, aryl, aralkyl or alkylaryl radicals which contain not more than 10 carbon atoms. However, these relatively thin suspensions containing only 56% of solid material display viscosities of 55 to 160 mPas, and have not proved to be stable since they form sediments.
It has therefore been a problem to find other dispersion agents which, when the solid material concentrations are equal or higher, will provide low viscosities and higher stability. In particular, stabilities of at least 3 weeks and the redispersion of possible sediments are necessary.
Extensive investigations with various wetting agents, dispersion agents, viscosity-regulating materials and the like, either alone or in combination, have provided insufficient improvement over the prior art.
Surprisingly, it has now been found that a small group of surface-active, anionic compounds is especially well-suited for the stabilization of aqueous dispersions of gypsum.